1. Field of the Invention
The present invention relates to the field of aquaculture, in particular but not exclusively in seawater, an industrial sector in full development, especially for the production of sea bass, sea bream and turbot.
It is aimed in particular at closed circuit fish farming.
2. Description of the Related Art
Considering here the case of seawater fish farming, it is in fact known that seawater fish farms operate in an open circuit discharging very large quantities of contaminated water. Typical quantities of water needed and discharged are assessed at about 200 to 300 m3 water/kg of fish produced. It is then understood that depending on the site at which such farms are installed, increasing environmental constraints are now tending to require the treatment of discharged waters, or even an absolute limit on the amount of water discharged.
Such a technique of farming seawater fish in a closed circuit (where at least part of the seawater of the rearing pond or ponds is therefore recycled in order to purify it and reinject it into the pond in question), is unquestionably an emerging technique in full development, even if its control is, in practice, very difficult and precise since it requires specific treatments to control the pH, the oxygen content, the CO2 content, the microbial and particulate load, the nitrogen load (especially aqueous ammonia), the dissolved contaminants, or even the color of the water in the ponds.
The fact remains that depending on the fish farmers, the control of this technique of closed circuit farming may enable, depending on the case, from 10 to 95 or even 98% of the seawater to be recycled, the rest being provided by new seawater makeup.
Nevertheless, it appears in practice, that although closed circuit water treatment circuits, which are quite complex, make it possible to control effectively parameters such as the pH, dissolved gases or even the microbial load of the recycled water (use of mechanical filters, of biological filters, of degassing columns, etc.), other problems are not yet truly resolved. Among these technical problems, which constitute real thorns in the life of these aquaculture plants, there is the problem of water coloration.
Specifically, the appearance of a yellow coloring of the water is observed, particularly at high recirculation rates (above 80%), which limits the visibility and unquestionably leads to stress in the farmed species.
Undoubtedly, the nature of these substances has not really been identified to date, but some works incline by towards the intervention of humic acids in this phenomenon. It is then found that no technology has really been validated in the current literature in order to resolve this problem, a problem which is most often termed xe2x80x9cyellow waterxe2x80x9d, UV light in particular having no effect on this color phenomenon.
One of the objects of the present invention is then especially to provide a method making it possible to improve the conditions in closed circuit fish farming, whether it involves seawater fish or freshwater fish or other sea products, aiming especially to demonstrate, on the phenomenon described above, a curative effect (in order to substantially remove the substances which would already have accumulated) and a preventive effect (in order to substantially limit the appearance of these coloring substances), while of course maintaining conditions acceptable for the farmed species in terms of toxicity and microbiology.
To simplify the terminology, in the following, the term xe2x80x9cfishxe2x80x9d will be used in the knowledge that it covers, according to the invention (unless otherwise specified) fish, eggs, seawater or freshwater young fish, or even sea products such as shellfish and mollusks.
The invention therefore relates to a method for improving conditions in closed circuit fish farming, in which:
a) at least part of the water of at least one rearing pond is recycled, recycled water which undergoes one or more purification treatments, before being reinjected into the pond in question;
b) in the recycling path of step a), the recycled water is mixed with new water makeup;
c) ozonation of all or part of the recycled water is carried out and in addition, if necessary, ozonation of the new water makeup is carried out before mixing it in step b.
The method for improving farming conditions according to the invention will also adopt one or more of the following characteristics:
the mixing of the recycled water and the new water makeup is carried out in a retention tank;
the new water makeup is ozonated by injecting a gas comprising ozone, so as to obtain an ozone content in the water of between 1 and 15 mg ozone/liter, and more preferably, between 2 and 10 mg ozone/liter of water;
the recycled water which is ozonated is ozonated by injecting a gas comprising ozone, so as to obtain an ozone content in the recycled water of between 0.05 and 2 mg ozone/liter, and more preferably between 0.1 and 1 or even 0.5 mg ozone/liter of recirculated water;
ozonation of all or part of the recirculated water and of the new water makeup is carried out;
the recycling path comprises a purification treatment using a biological filter, and the gas comprising ozone is then injected upstream of this biological filter;
the recycling path comprises a purification treatment using a biological filter, and the gas comprising ozone is then injected downstream of this biological filter, before the recirculated water is mixed with the new water makeup;
ozonation of all or part of the recirculated water and of the new water makeup is carried out, and two ozonators are available for this purpose so that each prepares one of the two ozonation gases needed;
ozonation of all or part of the recirculated water and of the new water makeup is carried out, and a single ozonator is available for this purpose, operating according to the following procedures:
i) said ozonator produces a main flow of ozonated gas;
j) a predetermined portion of the main flow of ozonated gas is diverted to a calibrated orifice, in order to direct it toward the recirculated water stream to be ozonated;
k) the power of said ozonator is adjusted on the basis of the flow rate of new water to be ozonated.
ozonation of all or part of the recirculated water and of the new water makeup is carried out, and a single ozonator is available for this purpose, operating according to the following procedures:
i) said ozonator supplies a main flow of ozonated gas;
j) a predetermined portion of the main flow of ozonated gas is diverted through a calibrated orifice, in order to direct it toward the new water to be ozonated;
k) the power of said ozonator is adjusted on the basis of the flow rate of recirculated water to be ozonated.
the improved fish farming is closed circuit farming of seawater fish.
The invention also relates to a method for decoloring the water of a closed circuit fish farm, a farm of the type where at least part of the water of at least one rearing pond is recycled, recycled water which undergoes one or more purification treatments before being reinjected into the pond in question, while in the recycling path, the recycled water is mixed with new water makeup in a retention tank, characterized in that the ozonation of all or part of the recirculated water, and also if necessary the ozonation of the new water makeup, is carried out before it is mixed with the recycled water.
The decoloration method according to the present invention must be understood as aiming in particular to limit the phenomenon of the appearance of yellow water, and to substantially eliminate the substances which would have accumulated beforehand.
The decoloration method according to the invention will moreover be able to adopt one or more of the following characteristics:
mixing of the recycled water and the new water makeup is carried out in a retention tank;
the new water makeup is ozonated by injection of a gas comprising ozone, so as to obtain an ozone content in the water of between 1 and 15 mg ozone/liter, and more preferably between 2 and 10 mg ozone/liter of new water, while the recirculated water is ozonated by injection of a gas comprising ozone so as to obtain an ozone content in the recirculated water of between 0.05 and 2 mg ozone/liter, and more preferably between 0.1 and 0.5 mg ozone/liter of recirculated water;
the recycling path comprises a purification treatment using a biological filter, and the ozonated gas is then injected upstream of this biological filter;
the recycling path comprises a purification treatment using a biological filter, and the ozonated gas is then injected downstream of this biological filter, before the recirculated water is mixed with the new water makeup;
ozonation of all or part of the recirculated water and of the new water makeup is carried out, a single ozonator being available for this purpose, which operates according to the following procedures:
i) said ozonator produces a main flow of ozonated gas;
j) a predetermined portion of the main flow of ozonated gas is diverted through a calibrated orifice, in order to direct it toward the recirculated water to be ozonated;
k) the power of said ozonator is adjusted on the basis of the flow rate of new water to be ozonated.
ozonation of all or part of the recirculated water and of the new water makeup is carried out, a single ozonator being available for this purpose, which operates according to the following procedures:
i) said ozonator produces a main flow of ozonated gas;
j) a predetermined portion of the main flow of ozonated gas is diverted through a calibrated orifice, in order to direct it toward the new water to be ozonated;
k) the power of said ozonator is adjusted on the basis of the flow rate of recirculated water to be ozonated.
the fish farming treated in this way is closed circuit farming of seawater fish.